This invention relates to a detecting tape suited for optically detecting the presence of silane gas, which is one of gaseous hydrides, by examining a trace formed the reaction of silane gas with a coloring reagent placed on a cellulose tape.
The semi-conductor industry uses highly toxic gaseous hydrides such as phosphine (PH.sub.3), silane gas (SiH.sub.4), arsine (AsH.sub.3) and diborane (B.sub.2 H.sub.6). For the detection of their trace leakage, various types of gas detectors are employed. Coloring reagents bringing about optical changes on reaction with other substances are commonly preferred to electrochemical and semi-conductor gas sensors that do not have high enough sensitivity. By an integral effect, methods using a coloring reagent can produce a trace of reaction even with a target gas of low concentration if the sample size is increased. Checking the formed trace with an optical concentration meter provides a foolproof detection of gas leakage.
Silver nitrate is popularly used as a coloring reagent for this purpose. A hydride detecting tape consisting of a gas-permeable cellulose tape carrying silver nitrate thereon that forms a colloid of silver as a trace of reaction when coming in contact with a gaseous hydride for subsequent optical examination.
However, the hydride detecting tapes carrying silver nitrate must be kept in light-tight places because sliver nitrate has an extremely high light sensitivity. Even when such storage precaution is taken, silver nitrate becomes brown in as short a time as approximately 24 hours. A nitride detecting tape proposed in Japanese Provisional Patent Publication No. 99753 of 1983 has an improved light resistance. The light resistance is improved by minimizing the formation of silver colloid on exposure to light by adding a strong acid, such as nitric acid, to a cellulose tape impregnated with silver nitrate and a moisture absorbent. When kept in light-tight containers, this nitride detecting tape remains undiscolored for about half a year. However, nitric acid added for the improvement of light resistance deteriorates the reagent-carrying cellulose tape, with a resultant sharp decrease in its mechanical strength. This reduction in strength can lead to serious problems in automatic measuring apparatus incorporating gas sampling and optical concentration detecting devices. To achieve automatic measurement, a portion of the unused part of the gas detecting tape is fed into the measuring area for exposure at given intervals. Therefore, the detecting tape set in the measuring apparatus is usually wrapped around a pay-off reel and a take-up reel, with a portion of the tape being adapted to pass through the measuring area. On completion of one sampling, the pay-off reel feeds a given length of the tape into the measuring area. However, the large tensile force working thereon occasionally breaks the tape entailing an interruption of measurement.
To solve these problems, the inventor proposed a detecting tape having an improved light resistance obtained without impairing the mechanical strength of the reagent-carrying tape by adding para-toluenesulfonic acid instead of nitric acid (Japanese Provisional Patent Publication No. 275352 of 1990). Remaining undiscolored and losing no mechanical strength for long periods of time, this improved tape assures reliable detection on automatic measuring apparatus. Even this tape, however, suffers some discoloration when stored for long periods of time because highly light-sensitive silver nitrate is used as the coloring reagent. Therefore, it does not have high enough sensitivity to assure satisfactory detection of silane gas that does not exhibit as much coloring as other gaseous hydrides.